Add zarr target

[sandorkertesz]

This PR adds the "zarr" target. It has a limited scope: GRIB data first converted to Xarray then the Xarray.to_zarr() method is used to generate the target Zarr store. E.g.

import earthkit.data as ekd
ds = ekd.from_source("sample", "pl.grib")

# with these options each field will be a separate chunk
ds.to_target("zarr", 
             earthkit_to_xarray_kwargs={"chunks": {"forecast_reference_time": 1, 
                                                   "step": 1, 
                                                   "level": 1}},
             xarray_to_zarr_kwargs={"store": "_pl.zarr", "mode": "w"})


import zarr 
root = zarr.group("_pl.zarr")
root.tree()

/
├── forecast_reference_time (4,) int64
├── latitude (19,) float64
├── level (2,) int64
├── longitude (36,) float64
├── r (4, 2, 2, 19, 36) float64
├── step (2,) int64
└── t (4, 2, 2, 19, 36) float64

[codecov-commenter]

Codecov Report

❌ Patch coverage is 45.83333% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.70%. Comparing base (387ed5a) to head (9a1e918).
⚠️ Report is 322 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/targets/test_target_zarr.py	43.47%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #716      +/-   ##
===========================================
- Coverage    90.77%   90.70%   -0.08%     
===========================================
  Files          169      170       +1     
  Lines        12916    12936      +20     
  Branches       615      616       +1     
===========================================
+ Hits         11724    11733       +9     
- Misses        1003     1015      +12     
+ Partials       189      188       -1     

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